1. Field of the Invention
The present invention relates to phase lock loop control; and more particularly, to an improved phase lock loop control for the rapid switching from one lock frequency to another.
2. Description of the Prior Art
Modern radars require the use of coherent microwave signal generators capable of extremely rapid frequency switching; such as in the order of 4 microseconds, for example. Within such time period, not only must the device be generating the new frequency, but it must also be in a definite locked phase relationship with a reference frequency.
For applications requiring switching speeds in the order of milliseconds, for example, a phase lock loop utilizing slowly varying sweep voltages is practical. In this instance, the frequency of the oscillator to be locked is slowly swept with an analog voltage sweeper until it passes through the desired voltage to provide the frequency point at which phase lock occurs. The acquisition of the phase lock is monitored through the use of the In-phase (I) mixers and quadrature (Q) mixers. Phase lock occurs through the quadrature mixture resulting in a constant DC voltage at the associated (I) mixer. This DC voltage disables the sweeper to complete the acquisition when it reaches a certain threshold. To insure reliable acquisition, the oscillator must be swept through the desired frequency at a maximum rate. This rate must be such that a frequency range corresponding to the servo loop bandwidth (F.sub.L) is traversed during a time period of at least (N/F.sub.L), where (N) is considered to be in the neighborhood of ten. If the oscillator searches linearly over a band B during time T, then EQU (B/T) .ltoreq. (F.sub.L /N/F.sub.L)
or EQU T .gtoreq. BN/F.sub.L.sup.2
for example, if the search band is 100 mHz and the loop bandwidth is 1 mHz, it takes about 1 millisecond to search the band. This time can be shortened by the use of conventional pre-positioning and narrower search ranges. However, such pre-positioning and narrower search ranges often lead to a row of "pot" adjustments which, in fact, fail to decrease the speed of acquisition significantly without becoming vulnerable to voltage controlled oscillator frequency drift due to temperature and aging.
In order to increase the switching speed and the phase settling time, it has also been proposed to employ two sweeping rates. The initial sweep is very rapid and sweeps the voltage controlled oscillator to nearly its correct frequency and the second sweep is slower for completing the lock. The output of the final loop mixer is monitored by a low pass filter and voltage comparator. The filter impulse response is such that the rapidly decreasing linear frequency modulated waveform caused by the search is sensed in such a way that the search is disabled as the VCO passes through the desired frequency. Such a proposed system increases the switching speed and particularly reduces the phase settling time so that a lock occurs between frequencies in the neighborhood of forty microseconds, for example.
In both of the aforementioned analog switching techniques, the system is limited by the decision making processes; that is, the system has to determine the precise time at which the sweeping voltage passes through the desired frequency.
In contrast to the search and acquisition techniques of the previous phase locked loops when switching from one desired lock frequency to another, is the digital method of changing the input voltage to the VCO. It is a known fact, that phase lock occurs very rapidly, if upon changing the loop reference frequency, the oscillator coarse tuning voltage corresponds to the coarse tuning voltage of the desired lock frequency. In other words, acquisition occurs quickly only if the frequency errors are small compared to the loop bandwidth. However, such an ideal situation has not heretofore been feasible because of frequency drifts in microwave oscillators caused by temperature change and aging.
Thus, it is desirable to provide an improved digitally controlled microwave phase lock loop which eliminates the decision making processes associated with conventional analog circuits to provide very fast frequency switching and phase settling times, regardless of the temperature and aging of the particular apparatus.